He is expected to make the announcement Wednesday night and will endorse a candidate.

Santorum won the 2012 Iowa caucuses and ended that race with the second-most number of delegates to eventually GOP nominee Mitt Romney. But he was unable to capture any momentum this year, despite extensive barnstorming efforts in Iowa.

He is the third Republican presidential candidate to drop out after Monday’s caucuses. Mike Huckabee ended his campaign that night, and Rand Paul suspended his campaign Wednesday morning.

The Santorum sources did not say whom Santorum would endorse. When asked about a possible endorsement, Florida Sen. Marco Rubio said he hasn’t spoken to the former Pennsylvania senator on Wednesday.

“I think he’s fantastic,” Rubio told CNN’s Dana Bash.

Santorum faced a drastically different landscape this year than he did in 2012. A crowded field of 17 Republicans and lackluster early polling kept him off the main debate stage for each of the GOP debates. He also faced competition for the hearts evangelical voters, particularly from Ted Cruz.

His retail politics were also no match for the media-centric, playbook-defying campaign of Donald Trump, the brash billionaire who left little room for candidates to win a moment in the spotlight.

Still, Santorum’s campaign was poised to learn from elements that overshadowed his bid in 2012, when the Pennsylvanian focused on social issues almost to a fault. He made a pointed effort to not get dragged down by controversial comments about homosexuality — which included a comparison to bestiality — that helped hamstring his earlier bid.

Without abandoning his socially conservative views, Santorum devoted more time to talking about his economic plans to revitalize American manufacturing and leaned heavily on his foreign policy knowledge to make the case he could best serve as president in an age of heightened global threats from terrorist groups like ISIS.

Santorum will endorse a candidate for the nomination, two GOP sources said. He, along with Huckabee, previously appeared with Trump at the billionaire’s event for veterans that was held opposite the Fox News debate in Des Moines, Iowa, on Thursday.